The invention relates to a ball joint, particularly an axial joint, having a housing, a bearing shell which is arranged in the housing, and a ball pin with a ball head which is arranged in the bearing shell.
Generally in ball joints and particularly in axial ball joints, the problem arises that with increasing wear of the bearing shell, a play of the ball head in the housing can occur. This play is basically undesirable.
An axial ball joint according to the prior art is shown in FIGS. 1 and 3. In FIG. 1, the axial ball joint is shown in an installed state before the housing is closed, and in FIG. 3 the ball joint is shown in the fully installed state. A housing 10 can be seen, in which a bearing shell 12 is arranged. A ball head 14 of a ball pin 16 is inserted into the bearing shell. In the fully installed state of FIG. 3, the marginal region 18 of the housing is flanged inwards, so that the ball head 14 is held securely in the housing 10.
In FIG. 2 the characteristic is shown of displacements of the ball head 14 in the case of stress in the direction of arrow F. It can be seen that the curve of force over displacement of the ball head rises sharply, because the ball head can only be displaced when the bearing shell is compressed. A high force is necessary for this.
In FIG. 4a the curve is shown which is produced in the case of a closed, fully installed ball joint under stress in axial direction. As the ball head 14 is pressed firmly against the bearing shell when the housing 10 is closed, the bearing shell undergoes a high elastic compression in the contact region. Accordingly a very steep curve of force over displacement is the result. It can be seen here that the curve runs without discontinuity through the zero point, because the ball joint does not have any play.
In FIG. 4b the profile of force over displacement of the ball head is shown, which occurs in the case of a ball joint with intensive wear. The wear has led the wall thickness of the bearing shell 12 to be partially reduced in the contact region to the ball head through settlement and abrasion. The ball head 14 can therefore be moved about its central position substantially without force downwards and upwards, whilst after abutment against the bearing shell 12 it can only be displaced further when the material of the bearing shell is compressed elastically. Therefore, a discontinuity of the curve is produced in the region of the zero passage, i.e. on a reversal of the load direction, whilst the curve outside the discontinuity has approximately the same gradient as in a practically new ball joint.
It is clearly undesirable if a worn ball joint has play. On the one hand, the play leads to noises when the ball head is moved from one side of the bearing shell to the other when the load direction is reversed. On the other hand, the play leads to increased stresses of the ball joint and of the components coupled therewith when the load direction is reversed.
From EP 0 784 755 A1, and corresponding U.S. Pat. No. 5,851,082, both of which are incorporated by reference herein in entirety, a ball joint is known which attempts to eliminate the occurrence of play due to wear. In this ball joint, a rubber ring is arranged between the housing and the bearing shell, the ring being compressed when the ball joint is installed and thereby partially pressing the bearing shell elastically against the ball head. Even if the bearing shell becomes worn, it continues to lie against the ball head because of the elastic force exerted by the rubber ring. In this way it is prevented that a play occurs and the ball head can be moved free of force inside the housing. However, a disadvantage in the known ball joint is that an additional component is present in the form of the rubber ring, which must be produced, kept in readiness and installed. This leads to increased costs.